1. Field of the Invention
The present disclosure relates to a method of improving tumor diagnostic efficiency of multivalent ligands by regulating the stoichiometric ratio between inner surface functionalities and ligand moieties for tumor targeting, and the multivalent ligands for tumor diagnosis.
2. Description of the Related Art
With the rapid development of modern diagnostic medical imaging equipments, it has become essential to develop contrast media with excellent sensitivity, accuracy, and safety to treat cancer which is the most common intractable disease for the modern people. Examples of imaging techniques that are used for the diagnosis of tumor include magnetic resonance imaging (MRI), computed tomography (CT), ultrasound, and positron emission tomography (PET) and single-photon emission computerized tomography (SPECT which use radioisotopes. Among them, CT and MRI provide anatomical images of human bodies, whereas nuclear medicine imaging methods such as PET and SPECT provide functional images of human bodies (information on physiological and biochemical reactions in vivo) by administering the radiolabeled ligand into a living body which binds strongly to a specific region.
PET and SPECT are one of the most frequently used techniques in nuclear medicine molecular imaging. Nuclear medicine imaging is a method in which a reconstructed image is obtained by measuring the emitted radiation from the radiolabeled imaging agent that was administered in vivo. A gamma ray is used as the radiation in nuclear medicine imaging which has high penetration efficiency against the tissue layer to afford images of superior sensitivity ex vivo; however the image resolution is poor. PET and SPECT are classified according to the principle of generating gamma rays from the radioisotope. PET detects two gamma rays generated when the positron and electron undergo pair annihilation, while SPECT detects a photon released when a nuclide in the excited state is destroyed by using a gamma camera, where a three dimensional image can be obtained by rotating the camera for detection. PET and SPECT are often used to examine or monitor thyroid diseases, cerebral diseases, heart diseases, blood circulation, etc., and are the molecular imaging techniques which have been frequently used to date despite the danger of radiation exposure because of their advantages of possibly detecting early-stage tumors. Additionally, the radiolabeled compounds for in vivo injection to obtain nuclear medicine images can be used as radiotherapeutic agents as well as imaging probes. As such, it is very important to develop such compounds with improved safety and efficiency.
Generally, multivalent ligands contain two or more ligand moieties within a molecule that bind noncovalently to a specific receptor or substrate, and they have high avidity through the multivalent effect which is not present in the molecule that can only bind monovalently to a receptor. Accordingly, such multivalent ligands have been widely used in the development of therapeutic agents for the diagnosis and treatment of diseases where stronger binding of these agents through avidity to the receptors expressed on the cell surface becomes more advantageous. Recently, the research on multivalent ligands for tumor diagnosis based on various organic or inorganic macromolecules and nanoparticles has been actively pursued (non-patent documents 1 to 8).
Tumor cells are generally known to overexpress various receptors as compared to normal cells, and numerous studies have been conducted on the development of targeted nanocarriers for the diagnosis of diseases or drug delivery, which can bind specifically to such receptors. However, these receptors can be partially expressed in normal cells as well as the tumor cells, and thus the nanocarriers may be deposited near the normal cells to exhibit unwanted toxicity. In an effort to overcome these problems, recently the nanocarriers with two or more ligand moieties attached on their surface which can bind to the same type of individual receptors on the surface of the same tumor cell have been reported to improve the selectivity and binding affinity toward the tumor cell.
However, the above reports are limited in that it may be difficult to control the number of maximally attached ligands consistently due to the steric hindrance during the preparation, or not all of the ligand moieties on a multivalent ligand could participate in binding to the receptors even if the multivalent ligand has been made to contain maximally achievable number of ligand moieties because of the limiting factors such as the size of the core and the distance between receptors expressed on the same cell.
In this regard, in order to resolve the above-mentioned problems, instead of substituting the surface of the scaffold or the core with a maximum number of ligand moieties, which has been considered as the most efficient method, the inventors of the present invention completed the present invention by discovering the effect that the avidity has been significantly improved by regulating the stoichiometric ratio between inner surface functionalities and ligand moieties for tumor targeting by the following: Once a ligand moiety of the multivalent ligand binds to a specific receptor expressed on the surface of a tumor cell, the phenomenon of receptor clustering is initiated whereby the same type of receptors migrate toward the receptor bound to the ligand moiety, making it more favorable for the unassociated ligand moieties to engage in multivalent binding. Furthermore, the appropriately selected inner surface functionality which has been then positioned in closer proximity to the cell surface by the bound ligand moieties can interact attractively with the surface of a tumor cell to promote the additional binding of unassociated ligand moieties to the clustered receptors.